Prosthetic vascular graft with a pleated structure

ABSTRACT

A vascular graft having a pleated circumference accommodates blood pressure changes with minimal change in internal surface area. A highly compliant graft may be made from a wide variety of polymers including non-elastomeric materials.

This is a divisional of application(s) Ser. No. 08/113,542, filed onAug. 27, 1993, which is now a divisional of Ser. No. 07/914,648 filedJul. 15, 1992, now U.S. Pat. No. 5,282,847 which is a continuation ofSer. No. 07/662,667, filed Feb. 28, 1991, now abandoned.

FIELD OF THE INVENTION

This invention is directed to synthetic vascular grafts.

BACKGROUND OF THE INVENTION

Vascular grafts are currently used to augment or replace certaindiseased arteries. Operations for this purpose are commonly done bysurgeons who attach the graft to the side of the vessel proximal to thediseased region and again to the side of the vessel distal to thediseased region, thus bypassing the diseased region. Alternatively thegraft is joined to the ends of the vessel remaining. after removing thediseased portion. Clinically used artificial grafts have generally beenrestricted to vessel replacements requiring grafts of 6 mm and largerdiameter, e.g., abdominal aorta, and iliac, femoral, and poplitealarteries. Natural vessels taken from other parts of the patients body,e.g., saphenous vein in the leg or internal mammary artery, are with fewexceptions the choice for less than 6 mm diameter applications. Examplesof the clinically used artificial vessels for arteries of large diameterare, for instance, DeBakey artificial vessel made of woven Dacron®(trademark of DuPont) and Gore-Tex® (Gore Co., Ltd., U.S.A.) which ismade of expanded polytetrafiuoroethylene (hereinafter "EPTFE").

According to U.S. Pat. No. 4,834,746 the prior Dacron® and EPTFE graftmaterials have proven unsuitable at diameters of less than about 6 mmbecause of the compliance mismatch between the natural and artificialvessels. The proposed solution in U.S. Pat. No. 4,834,746 is to producea vessel made of a microporous elastomer material. Etastomericmaterials, however, lose wall thickness in the process of expansion andtherefore eventually can result in aneurysm. Thus a conventionalcircular perimeter graft which merely utilizes an elastomeric materialto provide a compliance match is not considered a fully satisfactorydesign.

Other variations in materials and configurations for vascular grafts areknown, for example from U.S. Pat. Nos. 4,892,539; 4,605,406; 4,941,870;4,759,757; 4,629,458; 4,300,224; and 4,647,416 as well as GB 2187463 andEP 0256748. However, there still exists a need for improved vasculargraft structure, particularly for graft structures which can be used atdiameters below 6 mm.

SUMMARY OF THE INVENTION

The invention provides a novel artificial tubular graft structurecharacterized by a pleated circumference. The new graft provides astructure with high compliance, resistance to aneurysm and a luminalsurface area which remains substantially unchanged during expansion.

There may also be a connection between stretching of the surface onwhich endothelial cells are resting and blockage (stenosis) of thevessel wall due to underlying smooth muscle cell proliferation(hyperplasia). An additional factor promoting stenosis through smoothmuscle cell hyperplasia may be the attachment of a non-compliant tube(the usual synthetic graft) to the compliant artery. Chronic flexing atthe anastomosis and/or flow disturbances may result from thisattachment, both of which could contribute to hyperplasia. If any ofthese connections are made in vivo, the inventive graft design,providing a compliant structure while minimizing wall stretch duringcyclic pulsatile expansion, will prove even more desirable.

Although the novel graft structure may be made of conventional materials(e.g. knitted or woven fabrics and EPTFE), solid wall constructions madefrom polymers which are relatively stiff compared to prior art materialscan also be used.

The inventive structures have good inherent kink resistance. Whereasconventional tubular grafts tend to develop a kink when bent around atight radius, the pleated tube of the invention tends to fold flat whenbent around a tight radius because pleats at opposite sides form naturalseams allowing the tube to lay flat rather than kink.

In an alternative embodiment at least some of the longitudinal pleatshave a wave-like or corrugated configuration in the longitudinaldirection. Preferably such longitudinally corrugated pleats areregularly spaced around the circumference between sections of straightpleats to provide the graft with even greater kink resistance.Additional kink resistance may alternatively be provided by providingthe pleats with a twist about the longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of the one embodiment of thevascular graft of the invention.

FIG. 2 is a fragmentary cross-section taken across the axis of anextrusion die used to form the graft of FIG. 1.

FIG. 3 is a sectional view showing a preferred form of the inventioninflated at the lowest normal physiological pressure.

FIG. 4 is a sectional view of the graft of FIG. 3 shown when subjectedto the highest normal physiological pressure.

FIG. 5 is a fragmentary perspective view of an alternate embodiment ofthe invention in which the pleats are twisted along the axis of thegraft to provide improved kink resistance.

FIG. 6 is a fragmentary perspective view of a second alternateembodiment of the invention in which some of the pleats are corrugatedfor improved kink resistance.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 there is shown an embodiment of the vascular graft of theinvention in its simplest form. The graft 10 has a pleatedcircumference, the individual pleats 12 extending longitudinally alongand parallel to the central axis of the tube.

The term "apparent diameter" when used herein to describe a pleatedvessel of the invention refers to the diameter of a circlecircumscribing the outer pleat tips.

The graft may suitably be formed by extrusion from a die 20 as shown inFIG. 2, followed by a drawing down of the extruded tube to reduce itsdiameter and wall thickness. The die 20 comprises a toothed core 22 anda mating jacket 24 separated by an extrusion gap 26. A central hole 27in core 22 allows for inflation of the extruded tube with air or othersuitable fluid during draw down to prevent collapse of the tubularstructure. An extrusion die as shown in FIG. 2 having 15 teeth about itscircumference, an apparent diameter of 8.15 mm, an extrusion gap 26 of0.38 mm, and a pleat depth (tip 28 to valley 29) of 0.69 mm has beenfound to produce acceptable 7 mm or smaller apparent diameter graftmaterials with several polymers such as Kraton™ G2075, a thermoplasticrubber sold by Shell Oil Co., Pellathane™2363-80A a polyurethane sold byDow Chemical Co., and C-Flex™ R70-001 EM50A, a styrene olefin copolymersold by the Concept Polymer Technology Inc. Blends of resins, such as5-25% polypropylene with Kraton G2075 or C-Flex™, or 1-5% K-Resin™ KR03,a styrene-butadiene copolymer sold by Phillips 66 Co., with C-Flex™ havealso been successfully employed. The C-Flex™ extrusions may readily bedrawn down to apparent diameters of 3 mm without tearing or loss ofpleated profile.

FIGS. 3 and 4 are sectional views of a preferred graft 10 shown as itappears when subjected to normal physiological blood pressures. As shownin FIG. 3, graft 10, having an apparent diameter A, is pleated and sizedto maintain its fully relaxed pleated form at the lowest normal bloodpressure. At the highest pressure, the cross-sectional area issubstantially increased due to unfolding of the pleats 12, providing thegraft with a new larger apparent diameter B. Preferably the graft 10remains slightly pleated at highest normal blood pressure, as shown inFIG. 4, so that the enlargement in cross-sectional area products littleor no change in wall perimeter dimension. Thus, under normal conditionsthere is little or no tensile stress on the graft wall due to normalpulsing blood flow therethrough. To achieve this property those skilledin the art will appreciate that the polymer selected and the graft wallthickness may be varied to provide a suitable flexural response to theblood pressure change while the number and depth of the pleats may bevaried to provide the desired volume differential between the relaxedand the fully expanded forms of the graft.

With the invention polymers of high tensile strength can be used sincenormal pressure differentials are accommodated in the flexural mode byunfolding of the pleats. Because most materials have a lower flexuralmodulus than tensile modulus, many materials that have goodbiocompatibility but were too stiff to use in prior vascular graftconfigurations can now be used in the inventive pleated structure.

Typical wall thicknesses range from 0.01" or less, suitably between0.002" and about 0.007". Desirably the number of pleats about thecircumference is at least 8, more preferably at least 15. Suitableexpansion ratios (i.e. increased cross-sectional areas of the graftbetween lowest and highest normal physiological pressures) are at least5%, preferably at least 25%, more preferably 40% or more.

For relatively stiff polymers, such as solid polypropylene orpolytetrafiuoroethylene, it is preferable to utilize a plurality ofnested thin walled tubes, desirably about 0.025 mm or less thick,optionally encasing a reinforcing layer, to produce a graft with gooddrape during handling and good suturability.

While the graft may be made from of a highly elastomeric polymer so thatthe circumference is expandable beyond the fully unfolded circumference,it is most preferred that the material have a low flexural modulus but arelatively high tensile strength. This is considered desirable toincrease the graft's resistance to aneurysm. If the pleated graft of theinvention were to expand beyond its designed operating limits and thepleated structure was fully unfolded, the stress on the wall would becompletely borne as a circumferential tensile stress. Because thetensile modulus is greater than the flexural modulus, expansion beyondthe design limit would result in a radially stiffer graft, moreresistant to aneurysm.

A further advantage of the invention is that the luminal surface arearemains substantially unchanged during normal expansion. As long as thepleated structure remains, the wall circumference, and hence the luminalsurface area, remains relatively unchanged during expansion of thegraft. In vitro studies have shown that endothelial and smooth musclecells (which line normal vessels and colonize graft materials, a processoften encouraged by cell seeding prior to implementation) modify theirbiological activity in response to stretching. In particular,endothelial cells increase production of endothelin a potent peptidehormone vasoconstrictor and smooth muscle cell chemotractant. It isbelieved that with a pleated vascular graft of the invention endothelialcells adhered thereto will produce relatively less endothelin than whenadhered to a non-pleated elastomeric graft, thereby lessening thetendency for hyperplasia to develop within or contiguous to theprosthetic graft.

A still further advantage of the invention is that a solid wall can beused. While the particular material chosen is not considered critical tothe invention, so that porous, woven or knitted fabric, or EPTFEmaterials may be used, graft porosity at least on the internal surfaceof the graft may be considered disadvantageous because:

(a) hemostasis may require precoating or preclotting;

(b) tissue ingrowth may result in thrombus formation at the luminal wallsurface;

(c) growth of an endothelial cell layer at the lumenal wall face may beinhibited by the roughness of a porous wall material; and

(d) wall degradation may be increased by allowing ingrowth ofinflammatory cells and an increased surface-to-volume ratio with porousmaterials.

The inventive grafts of FIG. 1 have some natural resistance to kinking,tending to flatten out but not completely close when sharply bentwithout internal pressurization. However, additional kink resistance canbe incorporated into the graft of the invention as shown in FIGS. 5 and6. In FIG. 5 a pleated graft 30 is shown having longitudinal pleats 32which are twisted so that they spiral around the axis of the tube ratherthan running parallel thereto. In FIG. 6 a graft 40 is shown in whichsome of the pleats are corrugated to provide a wave-like pattern whenviewed from the side. These corrugated pleats 42 may be uniformlydistributed around the circumference of the graft, between uncorrugatedpleats 44. Suitably one or two corrugated pleats 42 are separated by acluster of three or four uncorrugated pleats 44. Optionally all of thepleats may be corrugated. In alternative embodiments not shown, aside-to-side wavelike pattern for some or all of the pleats may beprovided.

Another modification of the inventive structure which may be employed isto provide a multi-layered wall of different polymer materials. Forinstance, the internal wall may be a very thin layer of a polymer suchas polystyrene which provides a good surface for endothelial cell growthwhile the outer layer may be of a porous material which allows the graftto be anchored in surrounding tissues. Such multilayered structures maybe provided by co-extrusion, by internal or external coatings on a graftwall, or by other known techniques for providing laminated or nestedpolymer structures.

A multi-layered structure would allow very thin walls to be used withsufficient strength due to the multiple layers. A thin wall profilewould also allow fabrication of the graft with more pleats in the walland/or deeper pleats.

Those skilled in the art will appreciate that the grafts of theinvention can be manufactured by many techniques. For instance, inaddition to those already described, the grafts may be manufactured byprecipitation from solution or emulsion, by vacuum forming thermoplasticor curing thermoset materials on a mandrel, or by solution extrusioninto an appropriate bath where the solvent would be extracted to providea reduced wall thickness graft. The techniques of U.S. Pat. No.4,605,406, U.S. Pat. No. 4,834,746 and U.S. Pat. No. 4,770,684, thedisclosures of which are incorporated herein by reference, may forinstance be readily adapted to manufacture grafts of the invention fromvarious polymer solutions. Other manufacturing techniques will bereadily apparent to those skilled in the art.

The vascular graft of the present invention may be used in the samemanner as a vascular graft of conventional design. For example, it maybe cut to a desired length and sutured to a blood vessel at its endsaccording to conventional vascular graft techniques.

What is claimed is:
 1. A vascular graft comprising:(a) a tubular wallstructure; (b) longitudinally extending pleats on the tubular wallstructure; the pleats comprised of a biocompatible polymer capable ofresponding flexurally to changes in blood pressure by providing thetubular wall structure with a first cross sectional area when maintainedat a low normal physiological blood pressure, and a second, greatercross sectional area when maintained at a high normal physiologicalblood pressure, while maintaining a constant interior wall perimeter forthe tubular wall structure.
 2. A vascular graft according to claim 1wherein the second, greater cross sectional area is at least 25% greaterthan the first cross sectional area.
 3. A vascular graft according toclaim 1 wherein the wall structure has at least 8 pleats.
 4. A vasculargraft according to claim 1 wherein the tubular wall structure is madefrom a biocompatible polymer selected from the goup consisting ofsilicone polymers, polytetrafluoroethylene, polyesters, polystyrene,polyurethanes, styrene-olefin copolymers, styrene block copolymers,polyethylene, and polypropylene.
 5. A vascular graft according to claim1 wherein the polymer flexural modulus is lower than its tensilemodulus.
 6. A vascular graft comprising:(a) a tubular wall structurewherein the wall thickness is at most 0.007 inch; (b) longitudinallyextending pleats on the tubular wall structure; the pleats comprised ofa biocompatible polymer capable of responding flexually to changes inblood pressure by providing the tubular wall structure with a firstcross sectional area when maintained at a low normal physiological bloodpressure, and a second, greater cross sectional area when maintained ata high normal physiological blood pressme, while maintaining a constantinterior wall perimeter for the tubular wall structure.
 7. A vasculargraft comprising:(a) a tubular wall structure; (b) longitudinallyextending pleats on the tubular wall structure, the pleats twisted aboutthe longitudinal axis of the tubular wall structure; the pleatscomprised of a biocompatible polymer capable of responding flexurally tochanges in blood pressure by providing the tubular wall structure with afirst cross sectional area when maintained at a low normal physiologicalblood pressure, and a second, greater cross sectional area whenmaintained at a high normal physiological blood pressure, whilemaintaining a constant interior wall perimeter for the tubular wallstructure.
 8. A vascular graft comprising:(a) a tubular wall structure;(b) longitudinally extending pleats on the tubular wall structure, aportion of the pleats corrugated in the longitudinal direction of thetubular wall structure; the pleats comprised of a biocompatible polymercapable of responding flexurally to changes in blood pressure byproviding the tubular wall structure with a first cross sectional areawhen maintained at a low normal physiological blood pressure, and asecond, greater cross sectional area when maintained at a high normalphysiological blood pressure, while maintaining a constant interior wallperimeter for the tubular wall structure.
 9. A vascular graftcomprising:(a) a tubular wall structure; (b) longitudinally extendingpleats on the tubular wall structure; the pleats comprised of alongitudinally extending solid extrusion of a biocompatible polymer, thepleats capable of responding flexurally to changes in blood pressure byproviding the tubular wall structure with a first cross sectional areawhen maintained at a low normal physiological blood pressure, and asecond, greater cross sectional area when maintained at a high normalphysiological blood pressure, while maintaining a constant interior wallperimeter for the tubular wall structure.
 10. A vascular graft accordingto claim 9 wherein the wall structure has at least 8 pleats.
 11. Avascular graft according to claim 9 wherein the pleats are made from abiocompatible polymer selected from the group consisting of siliconepolymers, polytetrafluoroethylene, polyesters, polystyrene,polyurethanes, styrene-olefin copolymers, styrene block copolymers,polyethylene, and polypropylene.
 12. A vascular graft according to claim9 wherein the wall thickness is at most 0.007 inch.
 13. A vascular graftaccording to claim 9 wherein the pleats are twisted about thelongitudinal axis of the tubular wall structure.
 14. A vascular graftaccording to claim 9 wherein a portion of the pleats are corrugated inthe longitudinal direction of the tubular wall structure.
 15. A vasculargraft comprising:(a) a tubular wall structure; (b) longitudinallyextending pleats on the tubular wall structure; the pleats comprised ofa biocompatible polymer, the pleats having a flexural response such thatthe pleats are capable of opening and closing in response to changes inblood pressure by providing the tubular wall structure with a firstcross sectional area when maintained at a low normal physiological bloodpressure, and a second, greater cross sectional area while maintained ata high normal physiological blood pressure such that the pleats are notfully opened when maintained at the high normal physiological bloodpressure, the pleats maintaining a constant interior wall perimeter forthe tubular wall structure as the pleats open and close.
 16. A vasculargraft according to claim 15 wherein the second, greater cross sectionalarea is at least 25% greater than the first cross sectional area.
 17. Avascular graft according to claim 15 wherein the wall structure has atleast 8 pleats.
 18. A vascular graft according to claim 15 wherein thepleats are made from a biocompatible polymer selected from the groupconsisting of silicone polymers, polytetrafluoromethylene, polyesters,polystyrene, polyurethanes, styrene-olefin copolymers, styrene blockcopolymers, polyethylene, and polypropylene.
 19. A vascular graftaccording to claim 15 wherein the wall thickness is at most 0.007 inch.20. A vascular graft according to claim 15 wherein the pleats aretwisted about the longitudinal axis of the tubular wall structure.
 21. Avascular graft according to claim 15 wherein a portion of the pleats arecorrugated in the longitudinal direction of the tubular wall structure.22. A vascular graft according to claim 15 wherein the polymer flexuralmodulus is lower than its tensile modulus.